Superconductor Josephson junction comprising lanthanum aluminate

ABSTRACT

A lanthanum aluminate (LaAlO 3 ) substrate on which thin films of layered perovskite copper oxide superconductors are formed. Lanthanum aluminate, with a pseudo-cubic perovskite crystal structure, has a crystal structure and lattice constant that closely match the crystal structures and lattice constants of the layered perovskite superconductors. Therefore, it promotes epitaxial film growth of the superconductors, with the crystals being oriented in the proper direction for good superconductive electrical properties, such as a high critical current density. In addition, LaAlO 3  has good high frequency properties, such as a low loss tangent and low dielectric constant at superconductive temperatures. Finally, lanthanum aluminate does not significantly interact with the superconductors. Lanthanum aluminate can also used to form thin insulating films between the superconductor layers, which allows for the fabrication of a wide variety of superconductor circuit elements.

CROSS-REFERENCE

This application is a continuation of Ser. No. 07/803,935, filed Dec. 9, 1991, pending, entitled: HIGH FREQUENCY SUBSTRATE FOR THIN-FILM LAYERED PEROVSKITE SUPERCONDUCTORS, which is a divisional of Ser. No. 07/233,637 filed Aug. 18, 1988, entitled: HIGH-FREQUENCY SUBSTRATE MATERIAL FOR THIN-FILM LAYERED PEROVSKITE PHOTOCONDUCTORS (now U.S. Pat. No. 5,523,282).

BACKGROUND OF THE INVENTION

This invention relates generally to layered perovskite superconductors and, more particularly, to substrates on which layered perovskite superconductors are deposited to form high-frequency electronic devices and circuits.

Materials exhibiting superconductivity at temperatures above the advantageous liquid-nitrogen temperature of 77° K were discovered only recently and have triggered a world-wide explosion in scientific and technological research. The first material to exhibit superconductivity above the temperature of liquid nitrogen was an oxygen-depleted layered perovskite compound of yttrium, barium, copper and oxygen, identified by the formula Y₁ Ba₂ Cu₃ O₇. Since this discovery, other similar layered perovskite copper oxide compounds, identified by the formula R₁ Ba₂ Cu₃ O₇, where R is a rare earth element, have also been found to be superconductive at temperatures above the liquid-nitrogen temperature. This particular group of layered perovskite superconductors is commonly referred to as "1-2-3" compounds, because of the number of atoms of each metal element in the copper oxide compound.

Still other layered perovskite copper oxide compounds, with even higher critical temperatures (the temperature at which superconductivity occurs), have been more recently discovered. These newer compounds contain four metallic elements instead of the three metallic elements contained in the "1-2-3" compounds, and they do not contain a rare earth element. In place of the rare earth element, these newer compounds contain metals such as bismuth or thallium.

The major advantage of the layered perovskite superconductors is that superconductive temperatures can be maintained using liquid nitrogen, which is considerably less expensive and troublesome than using liquid helium, as required in the past. Therefore, these superconductors can be expected to find many new applications. One major application already being investigated is integrated circuits, in which thin films of these new superconductors are deposited on substrates to form, for example, Josephson junctions, waveguides and microwave transmission lines. These superconductor circuit elements can be combined to form high-speed, high-frequency and low-power integrated circuits with unparalleled performance.

However, thin films of the layered perovskite superconductors can only be grown with optimal properties on substrates having crystal structures and lattice constants that closely match those of the superconductors. Strontium titanate (SrTiO₃) is one such material, and is currently being used as a substrate. Unfortunately, strontium titanate is unsuitable at high frequencies because it is very lossy and has an extremely high dielectric constant at superconductive temperatures. Accordingly, there has been a need for a substrate material having good high frequency characteristics and a crystal structure and lattice constant that closely match the crystal structures and lattice constants of the layered perovskite superconductors. The present invention clearly fulfills this need.

SUMMARY OF THE INVENTION

The present invention resides in a substrate of lanthanum aluminate (LaAlO₃) on which thin films of layered perovskite copper oxide superconductors are formed. Lanthanum aluminate, with a pseudo-cubic perovskite crystal structure, has a crystal structure and lattice constant that closely match the crystal structures and lattice constants of the layered perovskite superconductors. Therefore, it promotes epitaxial film growth of the superconductors, with the crystals being oriented in the proper direction for good superconductive electrical properties, such as a high critical current density. In addition, LaAlO₃ has good high frequency properties, such as a low loss tangent and low dielectric constant at superconductive temperatures. Finally, lanthanum aluminate does not significantly interact with the superconductors. Lanthanum aluminate can also be used to form thin insulating films between the superconductor layers, which allows for the fabrication of a wide variety of superconductor circuit elements.

It will be appreciated from the foregoing that the present invention represents a significant advance in the field of superconductors. Other features and advantages of the present invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the crystalline structure of a lanthanum aluminate (LaAlO₃) substrate on which is deposited a thin film of a layered perovskite superconductor having the formula Y₁ Ba₂ Cu₃ O₇ ;

FIG. 2 illustrates the unit cell structure of lanthanum aluminate;

FIG. 3 is a fragmentary, sectional view of a Josephson junction in accordance with the present invention;

FIG. 4 is a fragmentary, sectional view of a coplanar waveguide in accordance with the present invention; and

FIG. 5 is a fragmentary, sectional view of a microstrip transmission line in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in the drawings for purposes of illustration, the present invention is embodied in a substrate of lanthanum aluminate (LaAlO₃) on which thin films of layered perovskite copper oxide superconductors are formed. Lanthanum aluminate, with a pseudo-cubic perovskite crystal structure, has a crystal structure and lattice constant that closely match the crystal structures and lattice constants of the layered perovskite superconductors. Therefore, it promotes epitaxial film growth of the superconductors, with the crystals being oriented in the proper direction for good superconductive electrical properties, such as a high; critical current density. In addition, LaAlO₃ has good high frequency properties, such as a low loss tangent and low dielectric constant at superconductive temperatures. Finally, lanthanum aluminate does not significantly interact with the superconductors. Lanthanum aluminate can also used to form thin insulating films between the superconductor layers, which allows for the fabrication of a wide variety of superconductor circuit elements.

FIG. 1 illustrates the structure of a lanthanum aluminate substrate 10 on which is deposited a thin film of a layered perovskite superconductor 12 having the formula Y₁ Ba₂ CU₃ O₇. As shown in FIGS. 1 and 2, each unit cell of lanthanum aluminate includes one atom of lanthanum 14, one atom of aluminum 16 and three atoms of oxygen 18. Although the unit cell is shown as including many more aluminum and oxygen atoms, the aluminum and oxygen atoms are actually shared with adjacent unit cells. As shown in FIG. 1, each unit cell of the layered perovskite copper oxide superconductor includes one atom of yttrium 20, two atoms of barium 22, three atoms of copper 24 and seven atoms of oxygen 18. Because the lattice constant of LaAlO₃ is approximately 3.80 angstroms and the lattice constant of the layered perovskite superconductor is approximately 3.85 angstroms, and because the crystalline structures of the two compounds are closely matched, the crystals of the layered perovskite superconductor orient themselves with the crystals of the lanthanum aluminate, thus providing high superconductive current flow in the direction of the arrows 26 shown in FIG. 1.

As shown in FIG. 1, the layered perovskite superconductor forms as layers of copper 24 and oxygen 18 atoms sandwiched between layers containing the other elements in the compound. Some of the copper-oxygen layers include planes of the atoms while other layers include chains of alternating copper and oxygen atoms. The layers containing the copper and oxygen atoms are the important layers for determining the superconductive electrical properties of the compound. Because the copper-oxygen layers are asymmetrically positioned in the unit cell, the compound is anisotropic in all of its electrical properties. This is why the ability of the superconductor to carry current is strongly dependent on its orientation.

The anisotropy of the layered perovskite superconductors is not the only problem caused by the complex chemistry and structure of these compounds. Each compound contains chemically reactive components, particularly barium, that strongly react with other substances. In addition, the compounds must be formed at very high temperatures, in the range of 700 to 950° C., to incorporate enough oxygen into the copper-oxygen layers to produce the proper crystalline structure. These high temperatures worsen the chemical reaction problems with the substrates upon which the thin films are deposited. One of the major advantages of LaAlO₃ as a substrate material is that it does not significantly interact with the superconductors. Furthermore, it must undergo a large amount of chemical substitution before it loses its insulating properties.

Another of the major advantages of LaAlO₃ is its high frequency characteristics. The dielectric constant of LaAlO₃ is less than 20, compared with 300 at room temperature and 18,000 at 4.2° K (liquid-helium temperature) for SrTiO₃. The loss tangent of LaAlO₃ is 8×10⁻⁵ at 77° K and 5×10⁻⁶ at 4.2° K, which is comparable to that of sapphire.

FIGS. 3, 4 and 5 illustrate the use of LaAlO₃ as a substrate and as an insulating layer in several microwave circuit elements fabricated from thin films of layered perovskite copper oxide superconductors. FIG. 3 illustrates a Josephson junction 30, FIG. 4 illustrates a coplanar waveguide 32 and FIG. 5 illustrates a microstrip transmission line 34. As shown in FIG. 3, the Josephson junction 30, which is the fundamental building block of superconductor electronics, includes a LaAlO₃ substrate 36, a thin film of layered perovskite superconductor 38 deposited on the substrate 36, a very thin insulating film 40 of LaAlO₃ deposited on the superconductor film 38, and another thin film of layered perovskite superconductor 42 deposited on the insulating film 40. The two superconductor films 38, 42 are the electrodes of the Josephson junction 30 and the insulating film 40 is the barrier through which tunneling occurs. In order for tunneling to occur, the insulating film 40 must be very thin, on the order of 20-30 angstroms.

As shown in FIG. 4, the coplanar waveguide 32 includes a LaAlO₃ substrate 44 on which are deposited a narrow thin film of layered perovskite superconductor 46 and two wide thin films of layered perovskite superconductor 48 on either side of the narrow superconductor film 46. The narrow superconductor film 46 is the conductor of the waveguide 32 and the two wide superconductor films 48 are the walls of the waveguide.

As shown in FIG. 5, the microstrip transmission line 34 includes a LaAlO₃ substrate 50, a thin film of layered perovskite superconductor 52 deposited on the substrate 50, a thin insulating film 54 of LaAlO₃ deposited on the superconductor film 52, and a narrow thin film of layered perovskite superconductor 56 deposited on the insulating film 54. The superconductor film 52 is the ground plane of the microstrip transmission line 34, the insulating film 54 is the dielectric and the superconductor film 56 is the conductor. In this device, the insulating film 54 is on the order of thousands of angstroms thick, rather than tens of angstroms, as in the Josephson junction 30. The microstrip transmission line 34 provides nearly dispersionless, low-loss transport of high-frequency electrical signals.

The thin insulating films of lanthanum aluminate and the thin films of the layered perovskite superconductors can be deposited onto the LaAlO₃ substrate by one of two basic processes, both of which are conventional. One of the processes starts with the superconductor compound and then deposits the compound by one of several methods onto the substrate. The other process starts with the constituent elements and actually forms the compound on the substrate. The first process is the easiest to perform, which starts with a pellet of the compound. The pellet is atomized in such a way that the liberated superconductor material lands on the substrate and forms a thin film coating. The pellet can be atomized using, for example, a laser (laser ablation), a stream of ions of a nonreactive gas such as argon (sputter deposition) or a vapor spray nozzle.

From the foregoing, it will be appreciated that the present invention represents a significant advance in the field of superconductors. Although several preferred embodiments of the invention have been shown and described, it will be apparent that other adaptations and modifications can be made without departing from the spirit and scope of the invention. For example, rare earth chromates and other rare earth aluminates having lattice constants within a few percent of the lattice constants of the layered perovskite superconductors should also be suitable substrates, providing the compounds do not significantly interact with the superconductors, the compounds have good high frequency characteristics and the compounds are non-ferromagnetic and non-ferroelectric. Accordingly, the invention is not to be limited, except as by the following claims. 

We claim:
 1. A superconductor Josephson junction comprising:(a) a crystalline lanthanum aluminate substrate; (b) a perovskite superconductor film on the lanthanum aluminate substrate; (c) a thin film of lanthanum aluminate deposited on the superconductor film; and (d) a perovskite superconductor film deposited on the thin film of lanthanum aluminate, each of the superconductor films having;(i) a crystalline structure that substantially matches the crystalline structure of lanthanum aluminate; (ii) a lattice constant that substantially matches the lattice constant of the lanthanum aluminate; and (iii) substantially no chemical interaction with the lanthanum aluminate; and, wherein said Josephson junction is capable of exhibiting superconducting electrical properties at temperatures maintainable with liquid nitrogen.
 2. The superconductor Josephson junction of claim 1, wherein the substrate and the superconductor films have perovskite crystalline structures that are substantially oriented to one another.
 3. In a superconductor Josephson junction in which two perovskite superconductor films are applied on a substrate, said superconductor films formed with a thin insulating layer therebetween, the improvement being the substrate and the insulating layer consists essentially of lanthanum aluminate having a crystalline structure and lattice constant that substantially match the crystalline structure and the lattice constant of the superconductor films, and wherein the crystalline structures are substantially oriented to one another, and there is substantially no chemical interaction between the superconductor films and the substrate.
 4. The superconductor Josephson junction of claim 3 wherein at least one of the superconductor films comprise erbium barium copper oxide superconductor formed on the substrate.
 5. The superconductor Josephson junction of claim 3 wherein at least one of the superconductor films comprise yttrium barium copper oxide superconductor formed on the substrate.
 6. The superconductor Josephson junction of claim 3 wherein at least one of the superconductor films comprise strontium, calcium, copper, oxygen, and an element selected from the group consisting of thallium and bismuth.
 7. The superconductor Josephson junction of claim 3 wherein the superconductor films comprise barium, calcium, copper, oxygen and thallium.
 8. A superconductor Josephson junction comprising:(a) a crystalline substrate of lanthanum aluminate having a predetermined lattice constant; (b) a first perovskite superconductor film comprising copper and oxygen, applied to the substrate; (c) a thin insulating layer consisting essentially of lanthanum aluminate applied to the first superconductor film; (d) a second perovskite superconductor film comprising copper and oxygen applied to the thin insulating layer the superconductor films having:(i) a crystalline structure that matches, and is substantially oriented to, the crystalline structure of the substrate; (ii) a lattice constant that substantially matches the lattice constant of the substrate; and, (iii) substantially no chemical interaction with the substrate.
 9. The superconductor Josephson junction as claimed in claim 8, wherein the superconductor films have superconductive electrical properties at the temperature of liquid nitrogen.
 10. The superconductor Josephson junction as claimed in claim 8 wherein the superconductor films (has) have a layered perovskite structure.
 11. A superconductor Josephson junction comprising:(a) a substrate and an insulating film each of lanthanum aluminate having a perovskite crystalline structure; (b) a perovskite superconductor film applied to said substrate and a perovskite superconductor film applied to said insulating film, the perovskite superconductor films having:(i) a second perovskite crystalline structure that substantially matches, and is substantially oriented to, the first perovskite crystalline structure of the lanthanum aluminate; (ii) substantially no chemical interaction with the lanthanum aluminate;whereby the perovskite superconductor films exhibit superconductor electrical properties at the temperature of liquid nitrogen.
 12. The superconductor Josephson junction as claimed in claim 11, wherein the first perovskite crystalline structure has a lattice constant ranging from about 3.80 Å to about 3.85 Å.
 13. A superconductor Josephson junction comprising:(a) a crystalline substrate of lanthanum aluminate having a predetermined lattice constant; (b) a first perovskite superconductor film applied to the substrate; (c) a thin insulating layer consisting essentially of lanthanum aluminate applied to the first superconductive film; (d) a second perovskite superconductor film applied to the thin insulating film, the superconductor films having:(i) a crystalline structure that matches and is substantially oriented to the crystalline structure of the substrate and insulating layer respectively; (ii) a lattice constant that substantially matches the lattice constant respectively of the substrate and insulating layer; and (iii) substantially no chemical interaction respectively with the substrate and insulating film. 